Defense cooperation is an important pillar of US-India strategic cooperation.Since the Modi government took office in 2014,US-India defense cooperation has entered the fast lane,and the two countries have made breakthroughs namely the signing of basic agreements,the transfer of high-precision and sensitive technologies,the joint research and development of advanced equipment,and interoperability exercises.US-India closer defense cooperation witnesses strategic convergence between the two sides,but the structural differences in ideology,strategic goals,domestic policies,and industrial bases between the two countries will limit the depth of cooperation.Therefore,the United States and India will not become military alliance in the full sense,but more likely a"quasi-alliance"that is"united but not aligned."This US-India defense cooperation has a strong orientation targeting China and its its military cooperation on the Sino-Indian border and the India Ocean poses tangible challenges to China's national security.